Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte between the anode and the cathode. The anode receives hydrogen-rich gas or pure hydrogen and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode, where the protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode are unable to pass through the electrolyte. Therefore, the electrons are directed through a load to perform work before they are sent to the cathode. The work may be used, for example, to operate a vehicle.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack includes a series of bipolar plates. The fuel cell bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates and cathode gas flow channels are provided on the cathode side of the bipolar plates. The fuel cell bipolar plates may also include flow channels for a cooling fluid.
The fuel cell bipolar plates are typically made of a conductive material, such as a carbon-composite or metal, so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. The fuel cell bipolar plates may be machined from relatively thin metal substrates or may be stamped to provide reactant gas flow fields and coolant fluid flow fields.
As is well understood in the art, a fuel cell needs to have a certain relative humidity. During operation of the fuel cell, moisture may enter the anode and cathode flow channels. At low cell power demands, typically below 0.2 A/cm2, water accumulates within the flow channels because the flow rate of the reactant gas is too low to force the water out of the channels. As the water accumulates, it forms droplets that continue to expand because of the hydrophobic nature of the plate material. The contact angle of the water droplets is generally about 90° in that the droplets form in the flow channels substantially perpendicular to the flow direction of the reactant gas. As the size of the droplets increases, the flow channel is closed off, and the reactant gas is diverted to other flow channels because the channels flow in parallel between common inlet and outlet manifolds. Because the reactant gas may not flow through a channel that is blocked with water, the reactant gas cannot force the water out of the channel. As more and more flow channels are blocked by water, the electricity produced by the fuel cell decreases. Because the fuel cells are electrically coupled in series, if one of the fuel cells stops performing, the entire fuel cell stack may stop performing. Further, water accumulation inside the channels on the anode side of the fuel cell may cause catalyst degradation on the cathode side of the fuel cell under certain conditions, which ultimately reduces stack performance and durability.
It is usually possible to purge the accumulated water in the flow channels by periodically forcing the reactant gas through the flow channels at a higher flow rate. However, on the cathode side, this increases the parasitic power applied to the air compressor, thereby reducing overall system efficiency. Moreover, there are many reasons not to use the hydrogen fuel as a purge gas, including reduced economy, reduced system efficiency and increased system complexity for treating elevated concentrations of hydrogen in the exhaust gas stream.
Reducing accumulated water in the channels can also be accomplished by reducing inlet humidification. However, it may be desirable to provide some relative humidity in the anode and cathode reactant gases so that the membrane in the fuel cells remains hydrated. A dry inlet gas has a drying effect on the membrane that could increase the cell's ionic resistance, and limit the membrane's long-term durability.